7. Alkenes: Reactions and Synthesis

Diverse Reactions of AlkenesAlkenes react with many electrophiles to give useful products by addition (often through special reagents)

alcohols (add H-OH)alkanes (add H-H)halohydrins (add HO-X)dihalides (add X-X)halides (add H-X)diols (add HO-OH)cyclopropane (add :CH2)

7.1 Preparation of Alkenes: A Preview of Elimination Reactions

Alkenes are commonly made byelimination of HX from alkyl halide (dehydrohalogenation)

Uses heat and KOHelimination of H-OH from an alcohol (dehydration)

require strong acids (sulfuric acid, 50 ºC)

7.2 Addition of Halogens to Alkenes

Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process

F2 is too reactive and I2 does not addCl2 reacts as Cl+ Cl-

Br2 is similar

Addition of Br2 to Cyclopentene

Addition is exclusively trans

+

Mechanism of Bromine AdditionBr+ adds to an alkene producing a cyclic ionBromonium ion, bromine shares charge with carbon

Gives trans addition

Bromonium Ion Mechanism

Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ionThis bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile

The Reality of Bromonium IonsBromonium were postulated more than 60 years ago to expain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkeneOlah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly

7.3 Halohydrin Formation

This is formally the addition of HO-X to an alkene (with +OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrinThe actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)

Mechanism of Formation of a Bromohydrin

Br2 forms bromonium ion, then water addsOrientation toward stable C+ speciesAromatic rings do not react

An Alternative to BromineBromine is a difficult reagent to use for this reactionN-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

7.4 Addition of Water to Alkenes: Oxymercuration

Hydration of an alkene is the addition of H-OH to to give an alcoholAcid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

Oxymercuration Intermediates

For laboratory-scale hydration of an alkeneUse mercuric acetate in THF followed by sodium borohydrideMarkovnikov orientation

via mercurinium ion

7.5 Addition of Water to Alkenes: Hydroboration

Herbert Brown (HB) invented hydroboration (HB)Borane (BH3) is electron deficient is a Lewis acidBorane adds to an alkene to give an organoborane

BH3 Is a Lewis Acid

Six electrons in outer shellCoordinates to oxygen electron pairs in ethers

Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylboraneOxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Hydroboration-Oxidation Forms an Alcohol from an Alkene

Orientation in Hydration via Hydroboration

Regiochemistry is opposite to Markovnikov orientationOH is added to carbon with most H’s

H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

Mechanism of Hydroboration

Borane is a Lewis acidAlkene is Lewis baseTransition state involves anionic development on BThe components of BH3are across C=C

Hydroboration: Orientation in Addition Step

Addition in least crowded orientation, synAddition also is via most stable carbocation

Hydroboration, Electronic Effects Give Non-Markovnikov

More stable carbocation is also consistent with stericpreferences

Hydroboration - Oxygen Insertion Step

H2O2, OH- inserts OH in place of BRetains syn orientation

7.6 Addition of Carbenes to Alkenes

The carbene functional group is “half of an alkene”Carbenes are electrically neutral with six electrons in the outer shellThey symmetrically across double bonds to form cyclopropanes

Formation of Dichlorocarbene

Base removes proton from chloroformStabilized carbanionremainsUnimolecularElimination of Cl- gives electron deficient species, dichlorocarbene

Simmons-Smith ReactionEquivalent of addition of CH2:Reaction of diiodomethane with zinc-copper alloy produces a carbenoid speciesForms cyclopropanes by cycloaddition

Reaction of Dichlorocarbene

Addition of dichlorocarbene is stereospecific cis

7.7 Reduction of Alkenes: Hydrogenation

Addition of H-H across C=CReduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2Hydrogen is first adsorbed on catalystReaction is heterogeneous (process is not in solution)

Hydrogen Addition- Selectivity

Selective for C=C. No reaction with C=O, C=NPolyunsaturated liquid oils become solidsIf one side is blocked, hydrogen adds to other

Mechanism of Catalytic Hydrogenation

Heterogeneous – reaction between phasesAddition of H-H is syn

7.8 Oxidation of Alkenes: Hydroxylation and Cleavage

Hydroxylation adds OH to each end of C=CCatalyzed by osmium tetroxideStereochemistry of addition is synProduct is a 1,2-dialcohol or diol (also called a glycol)

Osmium Tetroxide Catalyzed Formation of Diols

Hydroxylation - converts to syn-diolOsmium tetroxide, then sodium bisulfateVia cyclic osmate di-ester

Alkene Cleavage: OzoneOzone, O3, adds to alkenes to form molozonideReduce molozonide to obtain ketones and/or aldehydes

Examples of Ozonolysis of Alkenes

Used in determination of structure of an unknown alkene

Structure Elucidation With Ozone

Cleavage products reveal an alkene’s structure

Permangante Oxidation of Alkenes

Oxidizing reagents other than ozone also cleave alkenesPotassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

Cleavage of 1,2-diols

Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compindsSequence of diol formation with OsO4 followed by diolcleavage is a good alternative to ozonolysis

Mechanism of Periodic Acid Oxidation

Via cyclic periodate intermediate

7.9 Biological Alkene Addition Reactions

Living organisms convert organic molecules using enzymes as catalystsMany reactions are similar to organic chemistry conversions, except they occur in neutral waterUsually much specific for reactant and stereochemistry

Biological Hydration Example

Fumarate to malate catalyzed by fumaraseSpecific for trans isomerAddition of H, OH is anti

7.10 Addition of Radicals to Alkenes: Polymers

A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerizationAlkenes react with radical catalysts to undergo radical polymerizationEthylene is polymerized to poyethylene, for example

Free Radical Polymerization of Alkenes

Alkenes combine many times to give polymerReactivity induced by formation of free radicals

Free Radical Polymerization: Initiation

Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradicalmoleculeA bond is broken homolytically

Polymerization: Propagation

Radical from intiation adds to alkene to generate alkenederived radicalThis radical adds to another alkene, and so on many times

Polymerization: TerminationChain propagation ends when two radical chains combineNot controlled specifically but affected by reactivity and concentration

Other Polymers

Other alkenes give other common polymers

Unsymmetrical Monomers

If alkene is unsymmetrical, reaction is via more highly substituted radical

Chain Branching During Polymerization

During radical propagation chain can develop forks leading to branchingOne mechanism of branching is short chain branching in which an internal hydrogen is abstracted

Long Chain Branching

In long chains, a hydrogen from another chain is abstracted

Cationic Polymerization

Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations